This invention relates to a three-dimensional, conductive felt fabric having an electromagnetic conductivity gradient through its thickness. The present invention also relates to a method for producing such fabrics and their use as broadband microwave absorbers.
Electrically conductive fabrics have, in general, been known for some time. Such fabrics have been manufactured by mixing or blending a conductive powder with a polymer melt prior to extrusion of the polymer fibers from which the fabric is made. Such powders may include, for instance, carbon black, silver particles, or even silver- or gold-coated particles. Antistatic fabrics which conduct electricity can also be made by incorporating conductive fibers, e.g., carbon fibers, carbon-filled nylon or polyester fibers, or metal fibers such as stainless steel into yarns used to make such fabrics, or directly woven or knit into the fabric. Electrically or magnetically conductive polymers such as polypyrrole or polyanaline can also be incorporated into textiles so as to provide conductivity. Kuhn et al., U.S. Pat. No. 4,803,096, discloses electrically conductive textile materials made by depositing pre-polymer solutions of polypyrrole or polyanaline onto the textile surface to provide a uniform coating and then treating to complete formation of the polymer.
Electrically conductive textile materials exhibit characteristics which make them suitable for various uses such as antistatic garments, antistatic floor coverings, components in computers, and generally, as replacements for metallic conductors, or semiconductors, including such specific applications as, for example, batteries, photovoltaics, electrostatic dissipation and electromagnetic shielding, for example, as antistatic wrappings of electronic equipment.
Electronic devices, such as computers, may generate electromagnetic waves, which may resonate within their enclosure, to interfere with the electronic device itself, or be emitted through openings in the enclosure, to interfere with other electronic equipment. Such electromagnetic interference (EMI) can be problematic, for example, where electronic devices such as games, laptop computers, and cellular telephones operated by passengers interferes with avionics of commercial aircraft. It is known that certain conductive materials can be used to reduce EMI, e.g., a) rubber which contains conductive fillers, such as metals and carbon in the form of particles and fibers; and b) moldable polyurethane foams provided in the shape of three-dimensional articles, such as cones, which foams are coated with conductive polymer or carbon particles held in place with a suitable binder.
Known conductive materials used to reduce EMI can have significant drawbacks. Rubber articles are heavy and therefore unsuitable for many applications. Carbon or metal impregnated foams while lightweight, are friable and prone to flaking of their conductive coatings. The resulting flakes are electrically conductive and can thus short out equipment. The frangibility of rubbers and foams renders them unsuited for cutting or drilling holes to provide ventilation, equipment ports, or wire openings, inasmuch as such cutting or drilling creates edges which are brittle and crumble easily.
Reflection is enhanced at a boundary where electromagnetic energy passes from a medium of one conductivity into a medium of a different conductivity. Thus, the greater the transition in conductivity from air to the substrate, the more likely electromagnetic waves are reflected, rather than absorbed or transmitted.
The absorption of EMI by a substrate may be increased by providing a substrate with an electrical conductivity gradient in the Az≅ direction (in terms of three-dimensional Cartesian coordinates), i.e., through its Ashort≅ dimension or, more simply, the thickness or depth of the fabric substrate. Consequently, it would be desirable to provide a three-dimensional structure having a conductivity gradient along its thickness, in order to minimize reflection of electromagnetic energy and thereby enhance its absorption and/or transmission.
Pittman et al., U.S. Pat. No. 5,102,727 discloses an electrically conductive textile fabric which has a conductivity gradient, but the gradient is in one or more planar directions of the fabric, i.e., the Ax≅ and Ay≅ directions. The conductivity gradient is obtained by varying conductivity of yarns in the plane of the fabric by varying inherent conductivity of the yarn fibers, relative number of conductive to non-conductive filaments in a yarn, or the extent to which yarns are coated with conductive polymer.
Mammone et al., U.S. Statutory Invention Registration H1,523 discloses a method of making polymer films having a conductivity gradient across its thickness. The method can be used in preparing cast films for metallized or film foil capacitors, providing graded polymer conductivity which slowly decreases as a function of depth.
It would be useful to provide a readily made fabric having a relatively continuous conductivity gradient in its short dimension. It would also be desirable to provide a non-friable, die-cuttable, flame- or fire-resistant treatable, and/or colorable fabric which is capable of reducing EMI emissions by absorbing EMI, especially over many decades of microwave bandwidth.
The present invention relates to a conductive textile fabric comprising conductive fibers providing a conductivity gradient through its thickness. In one embodiment, the fabric of the invention comprises conductive fibers which are entangled.
The present invention further comprises a fabric which varies through its thickness in a property selected from the group consisting of intrinsic fiber conductivity, susceptibility to fiber coating by conductive materials, fabric density, fiber density, fiber denier, and fiber surface area. The fibers of the fabric of the present invention can be intrinsically conductive and/or can comprise a conductive coating.
The present invention further relates to a method for preparing a conductive textile fabric comprising conductive fibers which provide a conductivity gradient through its thickness which comprises:
a) providing a fabric comprising entangled conductive fibers which fabric varies through its thickness in intrinsic fiber conductivity, or
b) providing a fabric comprising entangled conductive and non-conductive fibers in which the percentage of conducting fiber varies through its thickness.
Alternatively, the present invention can relate to a method for preparing a conductive textile fabric comprising conductive fibers which provide a conductivity gradient through its thickness which comprises:
i) providing a fabric comprising entangled non-conductive fibers which fabric varies through its thickness in a property selected from the group consisting of susceptibility to fiber coating by conductive materials, fabric density, fiber density, fiber denier, and fiber surface area; and
ii) coating the fibers with a conductive coating selected from the group consisting of conductive polymer, metal coating, and carbon powder coating. This method can further comprise iii) additionally providing over the conductive coating a subsequent coating selected from the group consisting of conductive coating protective coating, flame and/or fire retardant coating, colorant coating and water repellent coating.
In another aspect, the present invention relates to a method for preparing an electromagnetically conductive textile fabric comprising conductive fibers which provide a conductivity gradient through its thickness which comprises:
1) providing a first web comprising entangled non-conductive fibers, said first web having a first density based on surface area of fibers per volume of said first web;
2) providing a second web comprising entangled non-conductive fibers, said second web having a second density based on surface area of fibers per volume of said second web, and said second web optionally comprising fibers which contain low temperature melting polymer;
3) providing an overlay comprising said first and second webs;
4) needlepunching said overlay to provide a unitary fabric, held together by the entanglement of fibers from the first and second web; and optionally heat-setting said unitary fabric;
5) coating the fibers of said first and second webs by contacting the fabric with a solution of conductive polymer precursor;
6) converting said precursor to conductive polymer to provide conductive fibers and a conductive gradient through the thickness of said fabric; and
7) optionally coating said conductive fibers with a flame retardant composition.
In yet another embodiment, the present invention relates to a method for reducing electromagnetic reflection of an electromagnetic radiation reflective surface which comprises covering said surface with an electrically conductive textile fabric comprising conductive fibers providing a conductivity gradient through its thickness.